My macbook pro osx10.7 has started to freeze when i try to restart. I think it started when i upgraded to Lion. After clicking on 'restart', the machine shuts off, restarts normally, goes to the gray screen with the apple logo, then goes to a black screen and freezes there. I have to power off, and start with the power button. The machine then starts normally. I have tried repairing disk permissions, about the extent of my expertise. Help please?